The present invention relates to radio communication systems, and more specifically, to frequency and/or channel selection using enhanced pathloss estimates.
Radio communication systems and cellular radio communication systems are typically allocated a specific band of frequencies to operate within, usually by a governmental body. Accordingly, operators of these communication systems desire to maximize the number of users who can communicate within the allocated bandwidth. A conventional method of maximizing system capacity is through frequency reuse. Frequency reuse is a technique whereby groups of frequencies are allocated for use in regions of limited geographic coverage known as cells. Cells containing the same or similar groups of frequencies are geographically separated to allow callers in different cells to simultaneously use the same frequency without interfering with each other. By so doing many thousands of subscribers may be served by a system of only several hundred frequencies.
Link quality is the benchmark of any radiocommunication system. To provide high quality voice communication the desired signal in a cellular system must maintain a minimum signal strength above all other interference. The ratio of the desired signal to the interference is known as the carrier to interference (C/I) ratio. Aside from noise, which is omnipresent, there are fundamentally two other types of interference with which a designer must contend. The first of these is interference arising from users simultaneously operating on the same channel. This is known as co-channel interference. The second source of interference is from users operating on adjacent channels. This is known as adjacent-channel interference. Adjacent channel interference is controlled by selecting the frequencies in a given cell to be separated by large frequency increments, e.g., in a typical GSM system a separation of 200 kHz between adjacent channels is used for frequencies allocated to one cell in a cell plan using three sector sites, and by using a sharp cutoff in the channel filters in order to obtain a high-adjacent channel suppression. Co-channel interference is reduced by use of a frequency reuse pattern which geographically separates cells with the same frequency group. An example of an ideal seven cell frequency reuse pattern is shown in FIG. 1.
Frequency planning is the process by which individual channels are assigned to cells within the network. Currently, most frequency planning is done a priori, i.e., a fixed frequency plan is xe2x80x9chard-wiredxe2x80x9d in place by each cellular system operator.
This is known as fixed channel allocation or FCA. However, as interference and traffic load are time varying, FCA is not optimal. For example, consider FIG. 2 which illustrates a highway which bisects a plurality of cellular boundaries. Since the highway may have significant automobile traffic in the morning and very little in the afternoon, the cellular traffic may significantly differ depending on location and time of day. As a result, most fixed frequency plans are not very efficient; many channels in a fixed frequency plan will have a much better link quality than is necessary to achieve high quality voice communication while many others in the same system will suffer from poor link quality which might force them to be dropped or blocked. A capacity increase could be obtained by some form of channel allocation which attempts to make the quality of all of the links equal, such as an adaptive channel allocation (ACA) scheme.
An important consideration in frequency allocation and cell planning is pathloss. Pathloss is a measure of the difference between the transmitted and received signal strengths. Various factors contribute to pathloss between the sending and receiving stations including the terrain, i.e., hills, trees, mountains, buildings. The greater the pathloss between the sending and receiving stations, the higher the transmitter power has to be in order for the receiving station to receive a signal of acceptable quality. Conventional systems for estimating pathloss for frequency planning purposes use propagation models to estimate the pathloss. The propagation models use information from terrain maps, actual site placements and antenna heights to estimate what the pathloss will be in the system. Based upon the estimated pathloss, which may or may not correlate to the actual pathloss in the system, the system determines the average disturbance in the cells and how much the different cells contribute to the disturbance level. The use of propagation models has serious drawbacks due to the fact that the models do not include the combined effects of the actual distribution of traffic and the actual radio propagation conditions.
Another solution for channel allocation is described in U.S. Pat. No. 5,491,837 to Haartsen, the disclosure of which is incorporated here by reference. In Haartsen, mobile stations are instructed to make received signal strength (RSSI) measurements of individual pilot signals transmitted from surrounding base stations. The RSSI measurements are used to estimate the pathloss between the base station whose signal was measured and the mobile station performing the measurements. However, the system of Haartsen has several limitations. The first limitation is that the RSSI measurements include signal energy associated with different types of interference, i.e., co-channel interference, adjacent channel interference, and interference from non-cellular emission (both licensed and unlicensed). Also, since the source of the interference is not known, the allocation of a new frequency may eliminate one type of interference, but the allocation may cause interference with other frequencies allotted to other users in other cells, which had good link quality prior to the new frequency allocation. Further, since Haartsen relies upon the detectors in mobile stations for measurements, and the quality of the detectors vary from one mobile station to the next, the measurements are susceptible to errors due to the different detectors.
The present invention relates to a method and system for frequency and/or channel allocation using enhanced pathloss estimates. According to exemplary embodiments of the present invention, available channels (e.g., frequency and time slot for a TDMA system, frequency, channelization codes and scrambling codes for a CDMA system) are allocated to a base station in order to serve a mobile station with the best compromise between the connection quality and the disturbance this connection will cause to other connections if a particular channel is used. Further, the channels are chosen based on the actual current usage of channels, power levels in cells and measured pathloss values between cells and mobile stations. Exemplary embodiments of the invention also provide estimates of the actual disturbance for both the uplink and downlink between a mobile station in one cell and base stations in other cells. Using the enhanced pathloss estimates the system also provides estimates of the impact due to changes in power level, changes in the amount of traffic and changes in the distribution of mobiles. Further, the system provides estimates of the effects of frequency replanning without causing disturbance to the system.
Through the use of the measurements provided by exemplary embodiments of the present invention, it is possible to optimize the frequency plan for a whole cellular communication system before the frequency plan is placed into operation. Exemplary embodiments of the present invention also use the measurements to optimize the frequency plan due to different traffic situations during a day, e.g., two frequency plans for an area one for high traffic and one for low traffic. In addition, the system uses the pathloss estimates in the determination of neighboring cells for handover execution, handover evaluation, and cell reselection.